The instant invention relates to soldering fluxes and solder pastes, and more particularly to an epoxy-based soldering flux which does not contain any volatile organic compounds or solvents (VOC's).
Soldering fluxes and solder pastes have heretofore been known in the art. In this regard, solder pastes are used to connect the leads of electronic elements to contact pads on a printed wiring board (PWB). In use, solder pastes are printed onto the copper contact pads of the PWB, the leads of the electronic element are placed in proper position on top of the PWB contact pads, and then the entire PWB assembly is heated to achieve reflow of the solder paste to form a connection between the leads of the electronic element and the contact pads of the PWB.
Solder paste, in general, comprises a homogenized mixture of solder powder and a soldering flux which, when mixed with the solder powder, gives the mixture the physical characteristics of a paste. The soldering flux is typically a solution of a resin, usually a rosin or rosin derivative, and an organic acid activator dissolved in a volatile organic solvent. The organic solvent typically comprises up to 80 wt % (percent by weight) of the flux composition. The organic acid activator in the soldering flux actively removes an oxide film from the metallic components when applied to the metal surfaces thereof, thus allowing the melted solder to flow freely over and bind to all fluxed surfaces. Thixotropic agents and other viscosity modifiers can also be present to achieve the desired rheological characteristics of the soldering flux and/or solder paste.
During the reflow, i.e. heating, process, most of the organic solvent evaporates into the air leaving a solid residue on the PWB. Furthermore, besides the evaporation of the organic solvents, a significant amount additional of organic material becomes airborne as fume through decomposition or sublimation of the solid residue. While the prior art solder pastes and fluxes have been highly effective for their intended purposes, the evaporation of the organic solvent and release of organic materials during solder reflow pose many problems which have recently become a significant concern in the industry. Firstly, volatile organic compounds (VOC's) and VOC emissions are now receiving stricter scrutiny from an environmental perspective. More stringent environmental regulations are requiring the reduction of organic emissions in all industrial applications. Secondly, VOC emissions are a burden to the reflow equipment in that the VOC's recondense onto the cold parts of the equipment, corroding metal, causing electrical failure, and clogging ventilation ducts. Another disadvantage to organic solvent based solder pastes is that in the case of rosin type pastes, the solid residue is brittle, and prone to cracking. Many manufacturers of printed wiring boards are now producing flexible circuit structures, i.e. flexi-circuits, and the brittleness and cracking of solvent based solder pastes is unacceptable in these types of applications. The use of plasticizers to remedy the brittleness of rosin-type organic solvent solder pastes makes the organic residue sticky, and unsuitable for pin-testing of the PWB after reflow.
The instant invention provides an epoxy-based, VOC-free soldering flux comprising 40 to 50 wt % (percent by weight) of epoxy resin, 25 to 30 wt % of non-volatile liquid epoxy diluent, 20 to 33 wt % of an activating agent which is insoluble, i.e. unreactive, in the epoxy system at room temperature, and 2.0 to 3.5 wt % of a thixotropic agent. In the instant soldering flux, the activating agent also serves as a curing agent for the epoxy resin system. There are no organic solvents in the soldering flux which can evaporate during the reflow process. The epoxy diluent can be either a reactive diluent containing one or more epoxy functional groups, such as polyglycol diepoxide, or a non-reactive diluent, such as a polyether diol. More specifically, the soldering flux preferably comprises 40 to 50 wt % of bisphenol A type epoxy resin, 25 to 35 wt % polyglycol diepoxide (epoxy diluent), 20 to 35 wt % of a polyfunctional carboxylic acid (activating agent and curing agent), such as malic acid, and 2.0 to 3.5 wt % of hydrogenated castor oil (thixotropic agent).
The instant invention further provides a VOC-free solder paste formed from the above-described flux consisting of 40 to 95 wt % soldering particles, and 5 to 60 wt % of the above-described VOC-free soldering flux. The soldering particles preferably comprise 63Sn/37Pb alloy solder particles having mesh size of about 325 to 500. The thixotropic agent added to the flux composition improves printability of the solder paste onto printed wiring boards. The activating agent of the flux effectively activates the solder powder and the copper contact during the reflow process, while at the same time cures the epoxy so that it hardens to leave a solid, dry, yet flexible residue on a printed wiring board. The activating/curing agent is selected so that its melts just prior to the reflow process so that activation, curing of the resin, and reflow of the solder particles occur almost simultaneously.
The instant invention still further provides a flux-core solder wire formed from the above-described VOC-free soldering flux. The flux-core solder wire is formed according to conventional solder wire forming techniques resulting in a wire structure having an inner core of the instant VOC-free soldering flux, and an outer shell of a solder alloy.
Accordingly, among the objects of the instant invention are the provision of an epoxy-based VOC-free soldering flux which hardens after the reflow process to form a solid, dry, flexible residue on the printed wiring board; the provision of a VOC-free solder paste; and the provision of a VOC-free flux-core solder wire.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds herein.